An airfoil is a part or surface, such as a wing, propeller blade, or rudder, whose shape and orientation control stability, direction, lift, thrust, or propulsion. Airflow immediately adjacent to, and influenced by, the airfoil is called the boundary layer. Airflow at the base of the boundary layer is in contact with the airfoil and not moving relative to it. By convention, the “top” of the boundary layer is the outer limit, not literally its “top” as opposed to “bottom”. The airflow gradually increases in speed until it reaches the free-stream velocity at the limit of the boundary layer. The boundary layer can be laminar (smooth) or turbulent. Laminar flow is generally limited to the forward portions of an airfoil surface, with the subsequent transition to a turbulent boundary layer occurring as a function of the flow's Reynolds Number (Re).
Various transducer devices measure the air velocity or air speed by constant temperature hot wire probes, or solid-state pressure sensors or others, and these devices generate an electrical signal as a function of the locally-sensed airflow velocity. The electrical signal (current, voltage, impedance, or resistance, depending on the system) has a steady-state component corresponding to average airflow speed and an overlaid oscillatory component corresponding to turbulence level. For ease of reference, these are referred to as DC and AC components, respectively, hereafter. The DC and AC components are electrically separated, then converted from analog to digital form separately by an analog to digital converter. These digital outputs are usually not linear functions of the sensed air velocities, and require further processing to linearize them before a comparison is performed between the AC and DC signals to produce a dimensionless Turbulence Intensity Ratio “R” which can be calibrated against actual performance at any suitable location on any given aircraft to provide a stall warning or investigatory system.
Such devices have performed successfully during extensive flight trials on numerous aircraft types, but they have exhibited one serious shortcoming: they fail to respond to severely separated airflow, in which the airflow proceeds from the back part of the airfoil to the front (“reverse flow”). Airfoils often exhibit reversed airflow near the stall and post-stall angles-of-attack. Areas of reversed flow are not registered by the equipment, resulting in an unusable or zero “R” values. The same may happen with distorted airflow at high sideslip angles.
In addition, it has also been observed that the use of a fixed R value can lead to a premature stall warning condition in the case of severe airfoil roughness, as might be caused by certain types of icing. The roughness causes a marked increase in the unsteady airflow component, which increases the R value and biases the stall warning level.
Therefore, there is a need to improve aerodynamic performance monitoring for airfoils.